Piston pipettes, which comprise a piston rod extending above the handle, are used for liquid dispensing, for example, in laboratories. The piston can thus be moved by the rod. Also, such pipettes are used in which the length of the intake movement of the piston can be set by rotating the rod. Such a pipette comprises also a volume display, typically based on digit rings, on which the set volume is displayed. One problem of these kinds of pipettes is that the rod may be accidentally rotated during pipetting, whereby also the set volume accidentally changes. In document WO 84/00119, a pipette is described in which for preventing accidental rotation there is a ring-like spring above the uppermost digit ring, the spring comprising at its lower surface protrusions and, corresponding to these, hollows on the upper surface of the ring. The rod can be rotated stepwise against the force of the spring.
The pipette according to document U.S. Pat. No. 4,909,991 comprises around the piston rod a flange non-rotatable with the rod. The body comprises above the flange a threaded sleeve, the upper end of which extends above the body. By rotating the sleeve, its lower end can be pressed tightly against the flange, whereby friction force prevents the accidental rotation of the rod.
The pipette according to document WO 91/16976 comprises a sleeve rotating with the piston rod and having longitudinal grooves on its outer surface. In the body there is a locking device comprising a spring and at its end a claw settling against a groove. The rod can be stepwise rotated against the force of the spring. The locking device can be, however, lifted up above the grooves, whereby the rod is freely rotatable. For this the locking device comprises an operation peg extending outside the body.